1. Field of the Invention
The present invention relates to scanning optical systems primarily for use in laser beam printers and the like, and more particularly to scanning optical systems having a tilt correcting function and adapted for use in printers including lasers of different wavelengths.
2. Description of the Related Art
In scanning optical systems for use in laser beam printers and the like, a deflecting device such as a polygonal mirror is used for deflecting the beam from a light source.
The deflection reflecting surface of the deflecting device has some tilt errors with respect to a direction orthogonal to the scanning plane which are attributable to manufacturing errors, installation errors or vibration during the rotation of the device.
When a beam is reflected from the deflection reflecting surface involving such tilt errors and projected onto the object to be scanned, images are formed thereon as deviated in the subscanning direction, consequently producing variations in the pitch of scan lines. These variations in the pitch of scan lines impair the quality of images to be recorded by recording apparatus such as laser beam printers.
Tilt correcting optical systems are adapted to eliminate the influence of tilt errors of the deflection reflecting surface to obviate this objection.
More specifically, the tilt correcting optical system has a linear image forming optical system for converging the beam from a light source in a direction orthogonal to the scanning plane first to form linear images on the deflection reflecting surface of the deflecting device, and an image forming optical system for forming a conjugate relation between the deflection reflecting surface and the surface of the object to be scanned. Accordingly, variations in the pitch of scan lines can be eliminated by using the correcting optical system.
In scanning optical systems having a polygonal mirror or like deflecting device, on the other hand, the speed of scanning of the object with a beam in the scanning plane is made constant.
Stated more specifically, the optical system is so designed that the angle of incidence at which the beam reflected from the deflection reflecting surface is incident on the system is made proportional to the image height when the optical system forms images on the surface of the object.
The term the "scanning plane" as used herein means a plane formed by a set of scanning beams in time series, i.e., a plane containing the main scan line on the object to be scanned and the optical axis of the tilt correcting scanning optical system.
However, to fully assure the beam of a constant scanning speed in the scanning plane, the problems of chromatic aberration, etc. need to be considered.
For example, with apparatus for forming color images on a silver salt film with three laser beams having wavelengths corresponding to red, green and blue colors, it is difficult to assure all the laser beams of a constant scanning speed on the surface to be scanned owing to chromatic aberration.
There are two kinds of chromatic aberrations, i.e., axial chromatic aberration and lateral chromatic aberration. Axial aberration means a difference in image forming position in the axial direction due to the difference in wavelength between laser beams.
In the event of lateral aberration, laser beams of different wavelengths form images at positions which differ in a direction orthogonal to the optical axis within the scanning plane, hence deviation of the beams in the scanning direction. The deviation of beams leads to an error in superimposing the spots of different colors and therefore exerts an adverse effect especially on images.
Alternatively in apparatus for recording images with a single laser beam, variations in temperature, for example, alter the wavelength of the laser beam, consequently making it impossible for the beam to scan the object at a fully constant speed. As a result, images of impaired quality will then be formed, for example, because spots will not be accurately positioned at image ends.